Laser amplifiers with high gain and small thermal aberrations

ABSTRACT

The present invention discloses a laser amplifier with high gain and low thermally induced optical aberrations on the amplified laser beam. The amplifier designs allow simple multipass configurations to optimally extract the gain and reduce thermally induced index of refraction aberrations, making it possible to obtain an amplified laser beam of high quality combined with very high overall gains comparable to those achievable with expensive regenerative amplifiers. The amplifier includes a thin active laser solid to create the population inversion and associated heat generation within the thin laser active solid possible for the desired gain value. The system includes a cooling device in thermal contact with the thin active laser solid to provide good heat transport and high reflectivity coatings at the wavelengths of the pump and laser wavelengths. The pump light sources are laser diodes tuned to the maximum absorption of the laser active material. The amplifier also includes an optical system to transport the pump light to the laser active solid in such a way as to further confine the absorption of light along the two orthogonal directions in the plane of the laser active solid in order to get high population inversion and consequently high gains possible.

CROSS REFERENCE TO RELATED APPLICATION

This patent application relates to, and claims the priority benefitfrom, U.S. Provisional Patent Application Ser. No. 60/642,112 filed onJan. 10, 2005 entitled LASER AMPLIFIERS WITH HIGH GAIN AND SMALL THERMALABERRATIONS, filed in English, and which is incorporated herein in itsentirety.

FIELD OF INVENTION

The present invention relates to solid state laser amplifiers and lasergain modules for high power laser resonators.

BACKGROUND OF INVENTION

Compact, efficient, high gain amplifiers and laser resonators are neededin a variety of applications in which one requires high output laserpowers in the smallest package possible. These applications include highpower free space amplifiers for fibre oscillators [1] and amplifiers forshort pulsed laser systems as a means to replace complex regenerativeamplifiers [2]. The use of high power and high gain amplifiers enables anew class of compact high peak power and high average power lasersources for material processing, laser manufacturing, and medical anddental applications for the highest processing speeds possible.

Solid state laser technology has now advanced up to the point where theaverage powers and peak power of pulsed sources has reached a thresholdwhere a number of new applications is now possible. These applicationshave been greatly limited in scope due to the lack of sufficientlycompact, low cost, robust laser system technology to move beyond thedemonstration phase. If the laser technology can be made robust and morecost effective a number of important applications are on the horizon.

Some of the most important applications involve pulsed laserapplications. It is under pulsed conditions that is possible to deliveryenergy to a material quickly, strongly localized the absorbed energy,and thereby raise the temperature of the material to its highest pointto drive ablation processes in reshaping the material to a desiredproduct [3]. The process of laser ablation and cutting, laser marking,etc. are more efficient the more strongly localized the energy is withinthe material both spatially and temporally.

There have been numerous methods developed over the years to achieve therequired power classes [4]. The challenge is achieving sufficiently highgain conditions as prescribed above without compromising laserbrightness. As laser gain media are pumped higher and higher to achievelarger gains, by whatever power source, there is an increase in thetemperature of the laser gain media as not all the power transferred tothe laser gain media is extracted by the laser beam. Some form ofcooling is required to prevent thermal damage of the laser gain media.The conditions of pumping and nonuniform cooling lead to a highlyaberrated thermal lens inside the gain media as a consequence of thetemperature dependence of the index of refraction [4, 5]. This is the socalled mirage effect; as things get hot they expand and the index ofrefraction decreases for most materials.

Due to the nonlinear conditions of laser amplification, these thermalaberrations act in a similarly nonlinear fashion to clamp the attainablelaser brightness for any given cooling/pumping condition. The laser beamquickly goes from diffraction limited TEM₀₀ mode to higher order modesthat can not be focused as tightly and thereby reduces the sourcebrightness as well as greatly reducing the working distance over whichthe beam can achieve its tightest focus. This problem is furtherexacerbated in the pursuit of high gain amplifiers where one must focusthe pump light to the smallest area possible to achieve the maximum gainper unit length for any given gain material. This latter condition canbe readily understood. There is a particular stimulated emission crosssection for any given atomic or molecular species that is responsiblefor the laser transition/laser action. By confining the excited speciesto the smallest area (largest number of excited states per unit area)the probability a photon from an incoming laser beam entering the gainmedium stimulating a photon emission event (and avalanche amplification)increases accordingly.

The conditions of confinement of the exciting pump light to achieve thehighest gain conditions increases the heat deposited per unit area andthe associated thermal gradients such that the problem of thermalaberrations becomes compounded. In addition to this consideration, underthe high gain conditions considered here, there is also the prospect ofspontaneous stimulated emission triggering a photon avalanche thatdepletes the gain by a process known as accumulated stimulated emissionor ASE [4]. This problem not only limits the extractable gain but alsodestroys pulse quality by adding long tails to the amplified pulse whichas explained above is highly deleterious to avoiding accumulated thermaldamage in laser processing.

The problem of ASE is most significant when the laser gain medium isbeing used as an amplifier; within a properly designed laser resonatorthese stimulated photons contribute to the laser beam rather than behaveparasitically. Optimal designs of high gain amplifiers/gain media mustspecifically address thermal aberrations and ASE issues from diminishingbeam quality and efficiency. Of these two issues, the thermalaberrations are most limiting with respect to power scaling and laserbrightness.

The various methodologies that have been developed to deal with thermalaberrations can be reduced to a few basic concepts employing two generalstrategies. One strategy is to try to minimize the magnitude of thethermal aberrations themselves for example by employing designs thatlead to faster heat transfer to keep the laser gain medium cooled [4],more uniform cooling to reduce index of refraction gradients in thetransverse direction of beam propagation so that all parts of the laserbeam experience the same index of refraction spatial profile [6,7], andthe use of cooling to reduce the magnitude of the change in the index ofrefraction with temperature by reducing the thermal expansion [8]. Theother strategy is to use beam configurations for sampling the pumpedgain region in such a way as to average out the thermal aberrations asmuch as possible such that all parts of the laser beam experience thesame spatial profile for the index of refraction and thereby removethermal lensing and aberration effects [9, 10].

Brauch et al. have disclosed the use of thin disc laser gain media inwhich thermal aberrations are removed for laser beams coming inperpendicular to a cooled surface [6]. Under uniform pumping conditions,there are no transverse components to the index of refraction spatialprofile experienced by the laser beam in the gain medium for thisconfiguration.

Wittrock [7] has used the concept of removal of thermal aberrations inwhich the laser beam to be amplified is brought in at a very small angleto the cooled surface, rather than normal to the surface as in theBrauch et al design [6], to enable cancellation of the thermal gradientsupon reflection as first described by Alcock and Bernard [10] in thegrazing incidence amplifier design. The relatively large acute angle tothe surface normal enables a much longer interaction region with thegain media and fewer passes to achieve the same overall gain as the thindisc concept of Brauch et al.

It is noted that both the Wittrock and Brauch et al. designs requireuniform pumping over the entire laser active solid to rigorously provide1 dimensional cooling. If uniform pumping is not accomplished theconcept fails to remove thermal aberrations. Furthermore, the conditionof uniform pumping greatly reduces the power extraction efficiency whilemaintaining high brightness for the laser beam to be amplified.

Laser amplifiers with lower gain require more round trips to extract thesame power as high gain systems and thereby experience more loss. Inprinciple, it is possible to have 100% percent reflectors such that alow gain system requiring up to 10 passes or more to extract power hasthe same efficiency as a much higher gain system that only requires onepass. As a matter of practice, high gain systems are essential to reducethe dimensions, number of optical surfaces involved in beam transportinto and out of the laser gain medium, and the overall path lengthtraversed by the laser beam to be amplified to attain maximum stabilityand efficiency.

The Wittrock design is capable of higher overall gain per pass relativeto the Brauch et al design, however it relies on completely uniformpumping of the laser active material. The laser beam to be amplified inthis design concept is brought in appreciably perpendicular to thenormal of the cooled surface such that the beam experiences allnonparallel components to the isotherms. In this case, the laser beam tobe amplified or resonator beam must be significantly smaller in diameterthan the uniformly pumped gain region to avoid diffraction losses andnonparallel isotherms from edge effects. The only way to increase thegain of the laser active solid in this design concept is to increase thepumping uniformly.

From a practical standpoint, the temperature rise in typical gain mediawith intimate contact with a heat sink reach elevated temperatures oftens to hundreds of degrees. The exposed surfaces with only air todissipate the heat get to extremely high temperatures under these samepumping conditions and thermally fracture. Surface heating is a wellknown effect that limits laser power and it is for this reason there arepatents explicitly covering the use of fused undoped end caps to reducesurface temperatures [11]. Diode pumped laser systems are designedspecifically to avoid pump light near surfaces or to use undoped endcaps to avoid surface heating and fracture for even modest laser powersand gains.

It would therefore be very advantageous to provide a laser amplifierwhich avoids the aforementioned difficulties and which can provide highgain and small thermal aberrations.

SUMMARY OF THE INVENTION

The present invention provides a solid state laser amplifier systemwhich overcomes these limitations in achieving high gain and beamquality by using a combination of thin laser gain media with laser beampropagation paths along different axes that substantially cancel thethermal aberrations and permit the use of nonuniform pumping for highgain.

In one aspect of the invention there is provided a solid-state laseramplifier system, comprising:

a) at least one laser-active solid having dimensions length L₁ width W₁,and thickness t₁;

b) a pumping light source;

c) light beam shaping optical system positioned adjacent to the pumpinglight source for shaping and directing a pump light beam from saidpumping light source into a first surface of said at least onelaser-active solid with an elliptical, round, or rectangular beam oflight with a length or long axis L₁′ and a width W₁′ satisfying acondition L₁′/W₁′≧1, and L₁′<L₁ and width W₁′<W₁, and wherein a regionof said at least one laser-active solid illuminated by the beam of lightproduces a pumped gain region defined by dimensions L₁′×W₁′×t₁ of saidat least one laser-active solid, and wherein t₁ is made as thin aspossible to most strongly localize the absorbed light and ensuing pumpedgain region that develops from pumping said at least one laser-activesolid with said pump light;

d) a cooling device, wherein the laser-active solid is slab-shaped andis fixedly connected at a second surface thereof to the cooling device,and wherein a major portion of heat generated in the laser-active solidby the pump light is removed by the cooling device to cool the secondsurface of the laser-active solid; and

e) an optical system configured to bring the laser beam to be amplifiedinto the laser active solid at an angle to the normal to the firstsurface of the laser active solid to remove substantially nonparallelisotherms that arise from nonuniform pumping of the laser active solidby the pump light beam and cooling requirements to achieve high gainconditions for the laser beam.

DESCRIPTION OF DRAWINGS

The solid-state laser amplifier system produced according to the presentinvention will now be described, by way of example only, reference beingmade to the accompanying drawings, in which:

FIG. 1( a) is a top view showing a solid-state laser amplifier systemwith a diode laser array pump light source;

FIG. 1( b) is a side view of the laser amplifier of FIG. 1( a) in whichdetails of the system when used with vertically stacked diode array canbe visualized;

FIG. 1( c) shows a side view of an embodiment of an amplifier whichincreases pump absorption by a multipass optical system;

FIG. 1( d) shows an embodiment of a solid state laser amplifier withcryogenic cooling;

FIG. 2( a) shows part of the laser amplifier showing the laser activesolid and laser inactive solid and dimensions used for these components;

FIG. 2( b) is similar to FIG. 2( a) but a different configuration of thelaser inactive solid coupled to the laser active solid;

FIG. 3 shows temperature and gain profile in the laser gain region ofthe solid state laser amplifier along x, y, and z direction;

FIG. 4( a) shows an embodiment of a solid state laser amplifier with afiber coupled laser diode with separated pump beam and seed laser;

FIG. 4( b) shows an embodiment of a solid state laser amplifier with afiber coupled laser diode with spatially co-propagating pump and seedlaser beams in a multipass configuration;

FIG. 5 shows an embodiment of a solid state laser amplifier with a laserdiode stack plus lens duct. Inset: details of thin laser active solidwith a sandwich structure used for gain confinement;

FIG. 6( a) shows an embodiment of a solid state laser amplifier in whichthe laser beam follows a zig-zag path along z direction;

FIG. 6( b) shows an embodiment of a solid state laser amplifier in whichthe laser beam follows a higher order zig-zag path; and

FIG. 7 shows an embodiment of the laser gain medium used in the solidstate laser amplifier which reduces or prevents ASE.

FIGS. 8 a and 8 b show another embodiment of a solid state laseramplifier according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a solid-state laser amplifier system foramplifying laser pulses. Basic elements of the solid-state laseramplifier system include: 1) a thin active laser solid of thickness t toabsorb light from an appropriate light source (the pump) to create thepopulation inversion and associated heat generation within the smallestvalue of t possible for the desired output power of the device; 2) apump source comprised of a laser diode or arrays of laser diodes tunedto the maximum absorption of the laser active atom, ion, or molecule; 3)an optical system to transport the pump light to the laser active solidin such a way as to further confine the absorption of light along thetwo orthogonal directions in the plane of the thin laser active solid;4) contact of the thin active laser solid to a cooling device with anappropriate material to give good heat transport and high reflectivityof light at the pump and laser wavelengths; 5) a cooling device thatmaintains the temperature of the active laser solid under pumpedconditions at the temperature at which the temperature dependentvariations of the index of refraction of the said active solid aresufficiently reduced to achieve the desired output power; 6) angularmultiplexing of the incoming laser beam to be amplified to canceltransverse components to the thermally induced index of refractionaberrations; and also to increase energy extraction from the said gainregion.

Preferred embodiments may also include an appropriate non-active lasersolid, typically the same as the host material used for the laser activesolid fused or bonded [13] or some other mechanical means to the laseractive solid, to provide mechanical support for the thin laser activesolid and which is shaped in such a way as to enable the coupling of theincoming and output beams within such a way for canceling the thermalaberrations and also to act as a heat buffer for the heat generated atthe surface of the said active solid and thus reducing thermalaberrations even further. The system may be designed to provide formultipasses of the pump beam through the laser active solid to increasethe amount of light absorbed in the desired volume or gain and toachieve an even greater degree of averaging out thermal induceddifferences in the index of refraction experience by the laser beam.

The system may also include periodically introduced absorber or otherloss structures along the pumped gain region of the laser active solidas needed to minimize ASE effects from depleting the stored gain in thelaser active media for obtaining higher gain in the amplification of thelaser beam. In addition, spatial filters may be used for the laser beamwhich are placed between subsequent passes of the laser beam beingamplified through the active solid in order to improve the beam qualityof the amplified laser beam. The system may also include a cryogeniccooling device to attain an operating temperature below a thresholdvalue, under pumped conditions within the laser active region, at whichpoint the index of refraction changes with temperature are negligiblysmall (near or at dn/dT=0 point) [8, 12, 14].

An important physical feature of this invention is the use of a verythin laser active solid. The dimensions of t₁ will typically be between10 microns and 500 microns, depending on the absorptivity of the laseractive solid at the pump wavelength. The dimension t₁ is made as smallas possible while still large enough to enable more than 50% absorptionof the pump light with the optical system used for transporting the pumplight. This value should be compared to more typical laser roddimensions of 5 mm; there is an order of magnitude reduction in thedimension of one principle dimension of the laser active solid overconventional laser rods. The use of a thin laser active solid is usedspecifically to confine the absorbed pump energy to ensure that any heat(q) generation occurs as close as possible to a heat sink. The time (τ)it takes heat generated in one region to undergo diffusive motion with adiffusion constant D to another region scales quadratically with respectto the length separating the two points (X), i.e. τ=X²/6D.

Thus, the rate of heat transfer to a heat sink scales inversely to thelength scale squared over which the heat is generated. The closer theheat can be generated to a heat sink the faster the heat can be removedfrom the material. The rate of heat transfer (dq/dt) is alsoproportional to the driving force in this case the difference intemperature (ΔT) between the heat sink and the hot object (dq/dt∝ΔT).For a fixed absorbed amount of pump laser energy, the thinner the lasergain medium the higher the temperature rise is under steady statepumping conditions. So the amount the heat transfer rate goes up morethan quadratically, the thinner the laser active solid is made.

The primary purpose of this invention is to make high gain, high poweramplifiers and laser gain modules so that high power solid state laserscan be reduced in dimension to their most compact robust dimensions. Thesurface heating effect is a major design consideration to meet thisobjective. The condition of uniform pumping to remove thermalaberrations imposes significant reductions in the achievable gain andextraction efficiency with respect to the present invention as detailedabove in the background material.

The problem with the thermal aberrations is handled in a new way thatenables the construction of the highest gain possible for a given pumpsource. The new concept of this invention combines essentially all theproven concepts for reducing the effect of thermal aberrations thatheretofore could only be introduced for the most part in isolation. Thisnew feature is enabled by the fact that the laser active solid or gainmedia in this invention is deliberately made as thin as possible in onedimension to enable simple focusing and angular multiplexing of bothpump and laser beam while still maintaining good spatial overlap of boththe pump and the laser beam undergoing amplification. The thin featureof the laser active solid in one dimension also facilitates the directintegration of absorbing material, saturable absorbers, and other lossmechanisms directly into the laser active solid to eliminate ASEproblems as much as possible.

The overall pumping system and layout for achieving these conditions isdescribed as follows. A side view of a first embodiment of a solid-statelaser amplifier system is shown generally at 10 in FIG. 1 a in which alaser diode pump source or laser diode array 12 is used in combinationwith an optical imaging system 14 and 16 to produce a beam of pump light18 to illuminate an elongated pumped region in the laser active solid orlaser gain medium 20. Here the optical imaging system is depicted as acombination of rod lens 14 and cylindrical lens 16 but is not restrictedto such.

The term “laser active solid” is used to mean the same thing as thephrase “laser gain medium”. These terms are meant to describe suitablydoped crystals such as Nd:YVO₄,Ti:sapphire, or other combinations oflaser active ions, atoms or molecules within a host matrix. A partiallist includes laser active ions based on various oxidation states of Nd,Ti, Cr, Er, W, doped in host crystals such as YAG, YVO₄, Sapphire,Fosterite to a suitable level to produce strong absorption at availablelaser diode wavelengths and gain at the desired laser wavelength so thatthe thickness of the laser active solid can be made as thin as possible.A typical laser active solid 20 for this application may be 1% dopedNd³⁺ in YVO₄.

A nonactive, transparent, solid support 22 through which the pump beam26 passes is shown. This feature provides mechanical support for betterhandling the thin laser active solid 20 in mounting and removes stressesfrom the contacted surface of the laser active solid 20 under nonuniformpumping. Equally important it provides a means to serve as a refractiveoptical element for permitting angular multiplexing of the laser beam 26to be amplified and the pump beam 18. This non-active solid support 22would typically be the undoped crystal host of the laser active material20 to provide good index of refraction matching across the interfaces toeliminate reflection losses as at the surface contacts.

Such a structure can be readily fabricated by diffusion bonding theundoped solid 22 to the doped laser active solid 20 at high temperatures[13]. Other transparent solids with better thermal and mechanicalproperties can also be used with appropriate mechanical fixtures to holdthe laser active solid 20 and the nonactive, transparent, solid support22 in optical contact. The pumping light source 12 includes a laserdiode bar, stack or diode array which are presently available withoutput powers in excess of 50 W. The heat deposited into thelaser-active solid 20 is removed from the laser-active solid by havingits lower surface in intimate thermal contact with a cooler 30 to befurther defined below.

Appropriate layers 32 are added to the surface of the laser active solid20 to achieve high reflection of both pump light beam 18 and the laserbeam 26 to be amplified and good thermal conductivity to the coolingdevice 30. One suitable series of layers to achieve this task is micronsthick SiO₂ layer 32 followed by a high reflection dielectric coating 34for the laser beam 26 and pump beam 18 wavelengths, followed by a heatconducting epoxy or indium solder 38 to make thermal contact. Theincoming laser beam 26 to be amplified by the laser active solid 20under pumped conditions is shown to come in at a glancing angleappropriate for grazing incidence amplification with removal of thermalaberrations in the direction of the pump beam 26. Other embodimentsshowing different beam geometries for improving this are shown in FIGS.4 and 5.

FIG. 1 b shows a view of a solid-state laser amplifier system 40 rotated90 degrees from the side view of amplifier 10 in FIG. 1 a in whichvertically stacked diode array 12 is shown combined through cylindricallens 16 to a common focus as in FIG. 1 a but with much higher pump lightpowers incident on the laser active solid 20 due to the verticallystacked diode array 12. Vertically stacked laser diode arrays are themost cost effective way to scale laser diode array pump sources tohigher powers. More diode arrays aligned in the same plane requirelonger laser gain media and becomes impractical for laser crystals morethan a couple of centimeters both for alignment issues and materialcosts in obtaining sufficiently long crystals of high quality. Inaddition, horizontally aligned diode arrays have much larger coolingcosts as each array requires an independent cooling system.

The ability to use vertically stacked diode arrays in this simple way isan important design concept of the present invention. The workingdistance over which vertically stacked arrays can be imaged to a focusto most strongly confine the absorbed pump light and achieve the highestgain is very limited. Vertically stacked laser diode arrays areavailable with pump powers in excess of 500 W and permit a factor ofmore than 10 scaling in output power. These extremely high power pumplaser sources are generally used for uniform pumping of large areas inwhich case the important directionality property of the laser radiationof the diode emitter is lost and the same pumping effect could beachieved with an incoherent light source. The use of highly doped thinlaser active solids to absorb the light within this narrowly definedregion enables simple optical systems to be used while maintaining highgain conditions.

FIG. 1 c shows a side view of system 40 to show how the pump laser beamscan be made to multipass the laser active solid for the more generalcase involving a vertically stacked laser diode array 12.

FIG. 1 d shows the details of the cooling device 30. In many casesnormal water cooling of a heat sink or thermoelectric coolers willsuffice for modest powers in the 1-20 W range of laser output. Thepreferred embodiment for maximum power and laser brightness is to use acryogenic cooler 30 in which a cooling fluid such as liquid nitrogen 50or other cryogenic fluid is used to drive the temperature of the laseractive solid 20 to the zero temperature dependent index of refractionpoint dn/dT=0 at which point all thermally induced aberrations areremoved. For certain materials there is a well defined temperature atwhich dn/dT=0. The temperature dependence is derived from thermalexpansion. By finding phase transitions at which point there is anextrema in the density of the material, one can achieve dn/dT=0conditions.

More generally, the thermal expansion coefficient decreases withdecreasing temperature as the lattice is made stiffer with decreasingtemperature and dn/dT approaches zero for all practical purposes over alarge temperature range. The wide temperature range is most ideal as thepumping conditions vary and change the temperature of the laser activesolid 20. This cooling and maintaining a temperature well below thatused conventionally permits a most general solution. This approach hasenabled an order of magnitude increase in output powers withoutdeterioration in the spatial quality of the laser beam to be amplifiedin bulk laser solids by using liquid N₂ at temperatures near 77° K. Thistemperature of the heat sink should remain constant to within ±10degrees to maintain these effective dn/dT=0 conditions within the laseractive solid. A suitable cryogenic cooling device is shown in FIG. 1 din which inlets for the cryogenic cooling liquid 50, vacuum insulatedwalls 52 surrounding the device and evacuated region 54 above the laseractive solid 20 with appropriate windows 56 for laser beam 26 and pumpbeam 18 inputs is shown.

FIGS. 2 a and 2 b show two different expanded views of the laser-activesolid 20 bonded to the transparent nonactive solid 22 to indicate thegeneral proportions of the two slabs 20 and 22 relative to one another.The dimensions L₁ refer to the length of the laser active solid 20 alongthe x direction, L_(1,2) to the common contact length between the laseractive solid 20 and transparent undoped material 22. Length L₂ refers tothe length of the free surface of the transparent solid region 22.Similarly W_(1,2) defines the width of the laser active solid 20 andtransparent solid 22 along the y direction. The height of the thin laseractive solid 20 is defined by to and that of the transparent solid byt₂. The pumped gain region 28 where most of the light from the pumpsource 12 is absorbed, exclusively in the laser active solid 20, isindicated by dashed lines. The dimensions to will vary between 10microns to 1 mm depending on the doping and absorptivity of the laseractive solid 20. The minimum thickness for absorbing >50% of the pumplight is used and this value depends on whether multiple passes of thepump are employed and gives rise to the stated range. For thicknessgreater than 1 mm, the thin active laser solid concept loses utility asthe heat transport to the cooling device 30, for typical laser materialthermal diffusivities, offer no major advantages over bulk laser gainmedia.

The preferred embodiment is to use 1-3% doped Nd3+:YVO₄ with a t₁ valueof less than 400 microns. The dimensions in the other directions dependon the power class and intended gain of the amplifier or gain module. Inall cases the For 20 W laser outputs based on 1% doped Nd^(3+:)YVO₄, theL₁′ dimension can be made between 1 mm and 1 cm and the W₁′ dimensionbetween 50 microns to 500 microns for small signal, single pass gains inexcess of 10. FIG. 2 a shows the general structure of a parallelepipedsuitable for grazing incidence amplification of the laser beam as ameans to reduce the thermal aberrations.

FIG. 2 b shows a different orientated trapezoid for creating zig zagbeam paths along the z direction for angular multiplexing of the laserbeam 26 to be amplified in such a way as to avoid the saturable absorberregions to be further described hereinafter. This embodiment in whichthe transparent solid region 22 is flipped over relative to itsorientation as drawn in FIG. 2( a) is used for creating zig zag pathsfor the laser beam 26 to be amplified.

FIG. 3 shows more detail with respect to the pumped gain region 28 andassociated temperature profiles that need to be cancelled through thedifferent laser beam propagation geometries. The pump laser source 12and optical system 14, 16 create an image within the laser active solid20 that leads to an elliptical irradiated laser gain zone 28 inwhich >50% of the light is absorbed with a long axis for the absorbedlight indicated as L₁′ in the x direction, width of W₁′ in the ydirection, and near uniform absorption extending the entire width of thelaser gain medium t₁ as shown in FIGS. 2( a) and 2(b). The relativedimensions of the pumped region 28 with respect to the laser activesolid 20 are as follows: L₁′/W₁′≧1, L₁′<L₁, and W₁′ less than orapproximately equal to W₁.

The length of the pumped region L₁′ is positioned to be between 100microns to 1 mm back from the laser beam input and output windows andsatisfy L₁′<L₁. This condition is relaxed along the y direction as thelaser pump light is generally intended to be spread out more along thetwo surfaces orthogonal to the laser beam and cooling device withL₁′/W₁′>2 such that W₁′ can be approximately W₁ if desired for a moreuniform temperature profile in this direction.

The length L₁, W₁, L₁′ and W₁′ are modified as needed for a particularpower class of amplifiers and gain. Larger dimension for L₁′ are used toincrease the power and smaller dimensions for W₁′ are used to increasethe gain per unit length along the x direction by appropriatemodification to the optical imaging system 14, 16 for the pump lightsource 12. The associated temperature profiles are schematicallydemonstrated in the lower half of FIG. 3. Near uniform intensityprofiles for the pump laser beam 18 along the L₁′ defined direction andgain saturation lead to a fairly flat temperature distribution along thelaser beam propagation direction and corresponding parallel isothermcomponents to the cooling device 30.

Similarly fairly uniform gain distribution are achieved along W₁ (y) andt₁ (z) directions with multipass pumping but the influence of thecooling direction modifies the temperature profiles as indicated. It isthe nonparallel isotherms in these respective directions that areremoved by cryogenic cooling and/or further reduced to acceptable levelsfor a given pump power by bringing the laser beam 26 in at grazingincidences to the cooling surface to minimize the thermal gradientsalong z. Smaller angles from the normal to the cooling surface, withmore reflections from the cooling surface (laser beam propagation in thexz or yz plane) will provide similar benefits. In like fashion, multiplebounces off the two adjacent surfaces normal to the surface of thecooling device 30, can be used to reduce thermal aberrations in the ydirection as needed for a given power class. Note, that the dimensionsof the gain media 28 (FIG. 2( a), 2(b)) in the y direction can also bemade to nearly match the pump width for near uniform pumping (W₁′˜W₁) tofurther minimize gradients. At pump powers in excess of 50 Watts, W1′will be on the order of 100 microns to 1 mm using the same preferredembodiment based on 1% doped Nd³⁺:YVO₄.

As described above, the trapezoid like structures shown in FIGS. 2( a)and 2(b) formed by the laser active solid 20 and laser inactive solid 22are designed for bringing the laser beam in preferred beam paths withrespect to the laser active solid 20 and transparent solid regions 22 toremove thermal aberrations. Higher order angle multiplexing can be doneto sweep out the gain and provide further averaging and cancellation ofthe thermal aberrations. Two such embodiments are shown in FIGS. 6( a)and 6(b). For geometrical reasons the configuration shown in FIG. 6( a)is simpler to implement with a cryogenic cooler 30. An elliptical laserbeam is then used in this few bounce embodiment to extract most of thegain along the L₁ direction. These higher order angle multiplexing laserbeam configurations are specifically exploited to simultaneously providebeam paths out of the surface plane in contact with the cooling device(within the xz plane) to deliberately avoid the absorbing channels (inthe xy plane) by the laser beam undergoing amplification to minimize ASEas will be detailed below.

Apart from laser diode arrays, other means to combine laser diode arrayscan be used to generate the approximately round, elliptical, orrectangular shaped pumped gain region as defined by dimensions L₁′ andW₁′ within the laser active solid 20. Referring to the embodiment of thelaser amplifier shown at 60 in FIG. 4( a) one can use a fibre coupledarray laser diode array 62 in which a lensing system including lenses 64and 65 areused to reflect the pump beam 67 back through the laser gainmedium 28 to give the same effective pumping geometry as obtained withthe free space focused laser diode arrays 12 of FIG. 1 a. A concavemirror 66 and a half-wave wavelength plate 68 are used in which thehalf-wave plate 68 rotates the polarization to improve the absorption inthe subsequent pass for media with strongly polarized transitions forabsorbing the light.

Alternatively, the difference in wavelength between the laser beam 26 tobe amplified and the pump laser diode arrays, along with the much higherbrightness of the fibre coupled laser diode arrays can be exploited toarrange a multipass system in which the pump laser light beam 18 andlaser beam 26 to be amplified propagate collinear. Other comparablebrightness sources such as multimode lasers can be used in an identicalfashion for pumping other gain material where diode lasers are notcurrently available. One important example is the use of frequencydoubled Nd³⁺:YAG lasers for pumping Ti:sapphire based amplifiers [8].All the same principles embodied above apply to other materials that usedifferent pump light source with similar coherence properties to laserdiodes.

The configuration as shown in FIG. 4( b) guarantees alignment of thespatial overlap between the pumped gain region 28 and the laser beam 26to be amplified for optimal alignment in which amplifier 70 includes afibre coupled laser diode array 72 which is directed to dichroic mirror74 which combines both pump laser beam 67 and the laser beam 26 to beamplified in a collinear fashion. Both the pump beam 67 and seed laserbeam 26 are brought in at an angle to the surface normal using a lens 76and is passed through the active laser gain media 28 and then reflectedback using a mirror 78 until the pump energy is optimally depleted fromthe pump beam 67 and the gain or power optimally extracted by the laserbeam 26. This arrangement guarantees spatial overlap between the pumpbeam 67 and laser beam 26 over the entire interaction path length. Inthe advent of very strong pump light requiring only a few passes, thepump beam 67 and laser beam 26 can be combined using separate opticalsystems rather than being made collinear.

The use of fibre coupled diode arrays as part of the overall inventionis advantageous as they produce output beams with circular crosssections that greatly facilitate the collimation of the pump beam thatpermit collinear beam propagation of both pump and laser beam for thesimplest possible alignment. The same concept for fibre coupled diodearrays producing round pump beams, as opposed to highly elliptical pumpbeams of lens coupled laser diode arrays, extends to all laser pumpsources of similar mode or higher brightness. Fibre coupled arrays alsoprovide a simple means to exchange diode laser arrays once the laserdiode array pump source is at the end of its lifetime and needs to bereplaced and therefore have great utility in implementing thisinvention.

The unique properties of thin gain media enable the use of lowbrightness imaging sources such as lens coupled diode arrays, high ordermode pump lasers, and fibre coupled diode arrays in multipassconfigurations for highest gain extraction efficiency. The highlyspatially confined laser gain media using the laser active solids of thetype described herein also permit the use of non-imaging optics forguiding the pump light to achieve high gain conditions that is theprimary focus of this invention. Since non-imaging optics such as lensducts are not capable of providing a focused images by definition, thepump light must be made to be confined in the smallest possible volumefor maximum gain by the structure of the doped laser active gain mediaitself.

This feature to the invention can be made to accommodate non-imagingoptics by bonding two transparent solids along the opposite facesperpendicular to the laser beam propagation and cooling surface. Thephysical dimension W₁′ is now the same as the same as W₁ of the laseractive solid region. The material bonded to the sides of the gain media20 should be undoped crystals of the same material as the host crystalfor proper index of refraction matching to avoid diffraction losses onthe laser beam best matched to extract the gain from this region. Thisfeature gives a flat temperature profile over the entire gain region asdepicted in FIG. 3 along the y direction and acts to further reducethermal aberrations. This design feature for high gain amplifiers withthin gain media is shown schematically in FIG. 5 for one such embodiment80 using a lens duct 82 which is used to direct the appropriate pumplight onto the laser active region 28 through a waveguide 84.

The surface of all the laser inactive solids may be coated with highreflection coatings such as to confine the pump light. In this case, thelaser inactive region 22 would be mechanically bonded rather than fusionbonded to simplify construction. The use of a sapphire clamping body aspreviously described by Miller et al. [15] would be particularlyappropriate in this application to both remove heat and eliminatesurface deformations and thermal fracture issues for the surface of thelaser active gain region not bonded to the cooling device.

The above describe generally how to strongly confine the absorbed pumplight into a very thin laser active solid in close contact with a heatsink for maximum power and gain for the most compact gain structurepossible prior to the onset of thermal fracture for a given power classand laser active material. This statement is made as thermaldiffusivities, thermal expansion coefficients and stress fracture limitsare all material dependent.

Overall for most materials, the gain that can be achieved with thesestructures becomes exceedingly high that accumulated stimulated emission(ASE) becomes the dominant factor limiting the achievable gain and powerfor a given structure. The problem of parasitic ASE losses are mostpronounced along the longest dimension of the pumped gain region L₁where the accumulated gain is largest. The very thin features of thegain medium 20 open up yet another parameter for further optimization tominimize ASE effects that have long plagued high gain amplifiers. Withsuch thin dimensions, one can use laser micromachining to drill outsmall slots in the laser active solid that extend throughout the entirethickness t₁ and are made to spatially coincide with the region L₁′×W₁′of the pumped gain region 28. The thin feature of the laser gain makesit possible to make slits 90 as shown in FIG. 7 as small as 1 to 10microns (depending on the thickness t₁) without incurring diffractionproblems that would limit the width of such slots for thicker materials.

These slots 90 form channels once the laser active material 20 is bondedto the transparent solid substrates and can be filled with stronglyabsorbing material to introduce loss periodically along the L₁′direction of maximum gain. Materials such as carbon black [16]interspersed in indium metal or specifically designed quantum dotsemiconductors to absorb only at the laser wavelength [17] suspended ina thermally conducting polymer matrix to act as saturable absorbers, canbe used as can any material that strongly absorbs at the laserwavelength and can be vapour sublimated to coat the surfaces within thechannel.

Air spaces with rough surface edges will also be useful to introducestrong loss modulation along the direction L₁. These slots 90 are placedat intervals equal to W₁ or less so that the ASE problems are no moreworse than the fundamental limit defined by the gain in the W₁direction. The total amount of lossy material introduced isinsignificant relative to the total gain volume, being less than 1% ofthe gain volume. Spontaneous emission with components in the x directiontraveling along the longest length of the gain region will beeffectively suppressed. The losses due to ASE will suppressed by afactor exp[g(L₁−W)] by this method where W is the spacing between thechannels and g is the effective gain per unit length in the small signallimit. This feature enables exponential contrast against ASE. Theproblem then reduces to directing the laser beam to be amplified along aspecific path that avoids these strong loss regions. In this regard itis important to have the ability to make the slits as small as possible.

FIGS. 8( a) and 8(b) show another embodiment of an amplifier which canbe built from commercially available components and simply aligned. Thepump source is a laser diode linear bar 118 with typical output powersin 20-60 W range. The pump light is directed and focused by an opticalsystem 120 which focuses the pump light 122 onto the thin solid activelaser material 104 creating the tight gain region 106. The typical sizeof gain region in the plane parallel to the active material surface isaround 1 mm. The optical system 120 can for example include twocylindrical lenses that focus the pump light in slowly and fastdiverging planes of the laser diode 118. It can also include a wedge forchanging the direction of the pump light so it hits the thin disk 102under a small angle so the back reflection of the unabsorbed pump light124 does not hit the laser diode 118 shown in FIG. 8 b. The unabsorbedpump light 124 can be disposed by a heat sink 126 or alternativelyrefocused and reflected back onto the gain region 106 to increase theoverall absorption. The thin active laser material 104 is attached on ametal heat sink 102 so the major heat flow from the gain region isapproximately perpendicular to the interface between the thin laseractive material 104 and the heat sink 102.

The laser beam to be amplified 108 is introduced into the amplifierapproximately collimated and focused on the gain region 106 with a lens110. The focused spot of laser beam 108 on the surface of the activelaser material 104 has approximately same size as the size of thefocused pump spot on the active material. After amplification andreflection from the back surface of the active material the laser beam108 is re-collimated by another lens 112 and back reflected by 180°folding prism. The laser beam is focused again by lens 112, amplifiedand reflected through the gain region 106 and re-collimated again by thelens 110. The laser beam 108 is subsequently reflected back by a mirroror preferably by another 180° folding prism in the plane perpendicularto the first folding plane so it will go through the gain region twomore times before it exits the amplifier as showed on the FIG. (8 a).

FIG. (8 b) is the side view of the described system and showing the pumplight 122 hitting the active material 104 under the small angle afterwhich the unabsorbed light 124 is collected by a heat sink 126. Thelight from laser diode bars is polarized so that polarization can beadjusted to maximize the absorption in the active material 104. Forexample, in case of Nd³⁺:YVO₄ with 1% doping as the active material andthe active material thickness 0.4 mm it is possible to achieve 80-90% ofabsorption in a single double pass. The amplifier performance can beimproved by attaching a non-active slab on the top of the activematerial 104 by diffusion bonding as discussed above. The fieldreplacement of the pump source can be simplified if high power fibercoupled laser diode is used instead of the laser diode bar. The beamquality of the amplified laser beam can be improved by inserting one ormore spatial filters for the laser beam 108 during its passes throughthe amplifier as known to the people skilled in the art. The number ofpasses through the gain medium can be easily increased by addingadditional prisms and mirrors. The amplifier can be folded by using twofocusing mirrors instead of lenses 110 and 112.

CONCLUSION

The use of thin laser active solid region is to increase the heattransfer rate to the heat sink for maximum output power, increase thegain per unit length for beam propagation in the plane of the laseractive solid, and to enable straightforward structuring of the gainregion to avoid ASE problems. The thin laser active solid has thesmallest dimension defined as t₁ for thickness and has a width W₁ andlength L₁. The typical dimensions of the laser active solid will have t₁values from 10 microns to 1 mm, L₁ values from 10 microns to severalcentimeters, and W₁ values from 10 microns to 1 cm. An undoped solidmaterial is bonded to the laser active solid to provide structuralsupport and to serve as a refractive optic for angular multiplexing thelaser beam to be amplified into the pumped gain region. This material istransparent at the pump wavelength to permit pump access to the laseractive solid and has dimensions L₂×W₂×t₂ where the dimensions arecomparable to the laser active medium but with t₂>>t₁.

As opposed to all other inventions that use thin laser active solidsunder uniform pumping conditions to create isotherms parallel to thesurface of the cooling device, this invention specifically employsnon-uniform pumping conditions to further confine the gain in the twodirections L and W that are orthogonal to the direction defined by itssmallest dimension t₁. This non-uniform pumping is accomplished by aoptical system for the diode laser pump source so as to define anelongated pumped gain region within the laser active gain media ofdimensions L₁′ and W₁′ in which L₁′<L₁ and W₁′<W₁. The thickness t₁ is acompromise between being as thin as possible to attain maximum heattransfer and absorbing sufficient pump light to maintain high efficiencyof the device.

The optical system enables multiple passes of the pump light in theregions L₁′ and W₁′ to enable thinner crystals to be used than singlepass pump configurations. The multipass feature also enables a degree ofgain saturation within the defined pump region to attain a condition ofparallel isotherms within said defined pumped area as defined bydimensions L₁′×W₁′ and of approximate thickness t₁. The non-uniformpumping is specifically carried out to increase the number density ofexcited states per unit area for maximum gain per unit length withoutintroducing surface heating effects and fracture of the uncooledsurfaces. The dimensions L₁′ and W₁′ are specifically chosen to matchthe incoming laser beam for optimal spatial overlap of the gain regionfor beam propagation along the length L without incurring diffractionlosses and maximum power transfer to said laser beam while maintaininghigh brightness.

The higher gain per unit length with this non-uniform pumpingarrangement leads to nonparallel isothermal components to the surfacewith thermal gradients along L, W, and t directions. These thermalgradients have associated index of refraction gradients due to the dn/dTdependence of the laser active solid and can lead to strong aberrationsof the laser beam to be amplified [5]. These effects are removed byusing at least one reflection off the interface in contact with thecooling device. This plane creates a unique inversion symmetry whichupon reflection, cancels the differences in index of refractionexperienced by different parts of the beam along the t direction.

An advantageous feature of this invention is the use of multipass opticsto enable the laser beam to undergo multiple reflections with the cooledsurface and further average out the effects of thermal aberrations onthe amplification process; while simultaneously increasing theextraction efficiency from the pumped laser gain region. The high gainfeature is also an important feature of this invention as the number ofsaid round trips for complete or saturated extraction of the gain isreduced to a few round trips such the total beam path through theaberrating medium is reduced as much as possible and in turn reduces theaccumulated affect of thermally induced the index of refractiongradients on the spatial profile of the amplified laser beam. In arelated embodiment, if the angle of the laser beam to the surface normalis made to be a glancing angle, the effects of thermal gradients alongthe W direction can also be removed if desired, while still maintaininggood spatial overlap with the gain region, by simultaneously reflectingoff the two orthogonal surfaces (L×t) multiple times in a zig zagfashion to spatially average out the transverse differences to the indexof refraction.

The beam is propagating in the L directions so gradients areinconsequential along this axis. For the highest power operation, thethermally induced aberrations are eliminated for all practical purposesby the use of cryogenic cooling to attain an operating temperature belowa threshold value, under pumped conditions within the laser activeregion, at which point the index of refraction changes with temperatureare negligibly small (near or at dn/dT=0 point). Temperature rather thanuniform pumping is used to generate an effectively spatially invariantindex of refraction throughout the laser active gain media. Any smallresidual aberrations can be completely removed with the anglemultiplexing of pump and laser beam as described above.

The thin feature of the gain media enable the straightforward imaging oflow brightness laser diodes, laser diode array, vertically stacked laserdiode arrays, or fibred coupled array pump sources to create cylindricalfocused pump beams to give the desired approximate L₁′×W₁′ pumped areathroughout the thickness of the laser active solid. These beams may berelay imaged back to strike the same region multiple times usingconventional combinations of lens, mirrors, and retroreflectors on themicro or macroscale. Alternatively nonimaging optics such as lens ductscan be used in which case the pumped region takes on the same values ofthe laser active solid (L₁, W₁, and t₁) with proper reflective andrefractive elements bonded to the laser active solid to confine thenonimaged pump light.

For a given W₁′ the overall gain of these amplifiers is increased byincreasing the value for L₁′ and permits scaling of the gain and averagepower for this class of amplifier modules. The gain eventually becomesso appreciable that ASE creates a serious problem for gain storage. Thisproblem is mitigated for the highest gain amplifier modules byperiodically imbedding strong absorbing centers or scattering centersalong the L or x direction, within the designed L₁′ pumped region, atintervals equal to or smaller than the width of the pumped region W₁′.The thickness of the imbedded absorbing material or scattering centersextends the full t₁ thickness of the laser active solid. This periodicloss blocks spontaneous emission from experiencing the highest gainalong the L₁′ or x direction which introduces a loss for the gainstorage that is exponentially larger by the difference L₁′-W₁′ than ASElosses along the W₁′ or y direction. The width of these loss centers canbe submicron to micron such that less than 1% of the potential gain islost in the process. Problems with ASE losses in the y and z directionsand back reflections in the x direction are handled by appropriateinclined angles to prevent back reflections, saturable absorbers alongthe laser beam propagation as needed to prevent back reflections seedingASE, and other measured employed by those skilled in the art.

As used herein, the terms “comprises” and “comprising” are to beconstrued as being inclusive and open ended, and not exclusive.Specifically, when used in this specification including claims, theterms “comprises” and “comprising” and variations thereof mean thespecified features, steps or components are included. These terms arenot to be interpreted to exclude the presence of other features, stepsor components.

The foregoing description of the preferred embodiments of the inventionhas been presented to illustrate the principles of the invention and notto limit the invention to the particular embodiment illustrated. It isintended that the scope of the invention be defined by all of theembodiments encompassed within the following claims and theirequivalents.

REFERENCES

-   [1] Tunnermann A, Hofer S, Liem S, Limpert J, Reich M, Roser F,    Schreiber T, Zelimer H, Peschel T, Guyenot V, “Power scaling of    high-power fiber lasers and amplifiers”, Laser Physics, 15 (1):    107-117 January 2005-   [2] Durfee C G, Backus S, Murnane M M, Kapteyn H C, “Design and    implementation of a TW-class high-average power laser system”,    IEEE J. Sel. Top. Quantum Electron., 4 (2): 395-406 March-April 1998-   [3] Vogel A, Venugopalan V, “Mechanisms of pulsed laser ablation of    biological tissues”, Chem. Rev. 103, 577 (2003)-   [4] W. Koechner, Solid-State Laser Engineering, 5^(th) edition,    Springer (1999)-   [5] Clarkson W A, “Thermal effects and their mitigation in    end-pumped solid-state lasers”, Journal of Physics D—Applied, 34    (16): 2381-2395, Aug. 21, 2001-   [6] Brauch, Uwe; Giesen, Adolf; Voss, Andreas; Wittig, Klaus; “Laser    amplifying system”, U.S. Pat. No. 5,553,088.-   [7] Wittrock, Ulrich; “Solid state laser amplifier”, U.S. Pat. No.    6,944,196-   [8] Backus S, Bartels R, Thompson S, et al., “High-efficiency,    single-stage 7-kHz high-average-power ultrafast laser system”, Opt.    Lett., 26 (7): 465-467, Apr. 1, 2001-   [9] Byer, Robert L.; “High power solid state laser”, U.S. Pat. No.    4,555,786-   [10] Alcock, Alfred J.; Bernard, John E., “High efficiency    transversely pumped solid-state slab laser”, U.S. Pat. No. 5,315,612-   [11] Beach, Raymond J.; Honea, Eric C.; Bibeau, Camille; Payne,    Stephen A.; Powell, Howard; Krupke, William F.; Sutton, Steven B.;    “High average power scaleable thin-disk laser”, U.S. Pat. No.    6,347,109-   [12] Brown D C, “The promise of cryogenic solid-state lasers”,    IEEE J. Sel. Top. Quantum Electron., 11 (3): 587-599 May-June 2005-   [13] St. Pierre R J, Mordaunt D W, Injeyan H, Berg J G, Hilyard R C,    Weber M E, Wickham M G, Harpole G M, Senn R, “Diode array pumped    kilowatt laser”, IEEE J. Sel. Top. Quantum Electron., 3 (1): 53-58,    February 1997-   [14] Ripin D J, Ochoa J R, Aggarwal R L, et al., “165-W    cryogenically cooled Yb: YAG laser”, Opt. Left., 29 (18), 2154-2156,    Sep. 15, 2004-   [15] Miller, Robert John Dwayne; Liao, Yan; Armstrong, Michael    Robert; Walker, David Ronald; “Laser clamping assembly and method”,    U.S. Pat. No. 6,385,220.-   [16] A. A. Oraevsky, L. B. Da Silva, A. M. Rubenchik, M. D.    Feit, M. E. Glinsky, M. D. Perry, B. M. Mammini, W. Small IV,    and B. C. Stuart, “Plasma mediated ablation of biological tissues    with nanosecond-to-femtosecond laser pulses: relative role of linear    and nonlinear absorption,” IEEE J. Sel. Top. Quantum Electron.,    2(4), 801-809 (1996)-   [17] Margaret A. Hines and Gregory D. Scholes, “Colloidal PbS    nanocrystals with size-tunable NIR Emission: Observation of    post-synthesis self-narrowing of the particle size distribution”,    Adv. Mater. 15, 1844 (2003).

1. A solid-state laser amplifier system, comprising: a) at least onelaser-active solid having dimensions length L₁, width W₁, and thicknesst₁; b) a pumping light source; c) light beam shaping optical systempositioned adjacent to the pumping light source for shaping anddirecting a pump light beam from said pumping light source into a firstsurface of said at least one laser-active solid with an elliptical,round, or rectangular beam of light with a length or long axis L₁′ and awidth W₁′ satisfying a condition L₁′/W₁′≧1, and L₁′<L₁ and width W₁′<W₁,and wherein a region of said at least one laser-active solid illuminatedby the beam of light produces a nonuniformly pumped gain region definedby dimensions L₁′×W₁′×t₁ of said at least one laser-active solid givingrise to nonparallel isotherms, and wherein t₁ is in a range from about10 microns to about 1 millimetre so as to most strongly localize theabsorbed light and ensuing pumped gain region that develops from pumpingsaid at least one laser-active solid with said pump light; d) a coolingdevice, wherein the laser-active solid is slab-shaped and is fixedlyconnected at a second surface thereof to the cooling device, and whereina major portion of heat generated in the laser-active solid by the pumplight is removed by the cooling device to cool the second surface of thelaser-active solid; and e) an optical system configured to bring thelaser beam to be amplified into the laser active solid, so that itoverlaps with the nonparallel isotherms that arise from nonuniformpumping, and is incident at an angle to a normal to the first surface ofthe laser active solid to substantially remove effects of thenonparallel isotherms that arise from nonuniform pumping of the laseractive solid by the pump light beam and cooling requirements to achievehigh gain conditions for the laser beam.
 2. The solid-state amplifiersystem according to claim 1 wherein said optical system and said laseractive material are configured to direct the laser beam to make at leasttwo passes through the laser gain medium to further average the thermalaberrations and increase energy extraction from the said gain volume. 3.The solid state amplifier system according to claim 1 in which thecooling device is a cryogenic cooler that maintains the temperature ofthe laser active solid under pumped conditions at a temperature at whicha differential change in index of refraction of the laser active solidwith temperature is substantially close to zero (dn/dT=0) to furtherreduce thermal aberrations.
 4. The solid-state laser amplifier systemaccording to claim 1 wherein the slab-shaped laser active solid includesperiodically disposed channels of length between about 0.1 micron toabout 100 microns, said channels being filled with an absorbing materialor scattering centers to introduce regions of absorption and scatteringlosses along a direction defined by L, and prevent accumulated stimulateemission along the L₁′ pumped region from depleting the gain in thisdirection and accompanying reduction in pulse quality for use in theamplification of laser pulses.
 5. The solid-state laser amplifier systemaccording to claim 4 wherein said channels are small cylinders or narrowlines that run parallel to a direction of W₁′ of the pumped gain medium.6. The solid-state laser amplifier system according to claim 4 whereinthe absorbing material are selected from the group consisting ofsemi-metals that strongly absorb at the laser wavelength, specificallytuned quantum dots tuned to act as saturable absorbers at the laserwavelength imbedded in an index matching polymer.
 7. The solid-statelaser amplifier system according to claim 4 wherein the scatteringcenters include air spaces or voids.
 8. The solid-state laser amplifiersystem according to claim 1 including a slab-shaped nonactive lasermaterial bonded to said first surface of said slab-shaped laser activesolid to give a composite structure to provide mechanical strength forthermal contact of said slab-shaped laser active solid to the coolingdevice, said slab-shaped nonactive laser material having an index ofrefraction for matching an index of refraction of said laser activesolid to avoid diffraction and Fresnel losses in coupling the laser beamto be amplified into and out of the pumped gain region.
 9. Thesolid-state laser amplifier system according to claim 8 wherein saidcomposite structure formed by the slab-shaped nonactive laser materialbonded to said first surface of the slab-shaped laser active solid isshaped as a trapezoid or parallelepiped with a pre-selected angle at theends of the composite structure into which the laser beam beingamplified is directed and an output from which the amplified laser beamexits from the composite structure, and wherein said optical system isconfigured for redirecting the amplified laser beam back through thepumped gain volume to substantially average out thermal aberrations forfurther increasing gain of the laser beam.
 10. The solid-state laseramplifier system according to claim 4 wherein said optical system isconfigured for directing the laser beam to be amplified into the pumpedgain region at one or more angles to generally avoid the regions ofabsorption and scattering losses as the laser beam passes through thepumped gain volume.
 11. The solid-state laser amplifier system accordingto claim 1 including a dielectric material located between the secondsurface of the laser active solid and the cooling device to giveincreased reflection of said laser beam to be amplified whilesimultaneously providing good thermal contact of said laser active solidto said cooling device.
 12. The solid-state laser amplifier systemaccording to claim 1 wherein said pumping light source is a laser diodearray.
 13. The solid-state laser amplifier system according to claim 1wherein said pumping light source is a vertical stack of linear laserdiode arrays.
 14. The solid-state laser amplifier system according toclaim 1 wherein said pumping light source is a fibre coupled laser diodearray or vertical stack of linear diode arrays.
 15. The solid-stateamplifier system according to claim 1 wherein the pumping light sourceis a laser pump source.
 16. The solid-state laser amplifier systemaccording to claim 1 wherein said optical system is configured fordirecting the laser beam to be amplified through the surface of thelaser-active solid along a direction which is substantially the same asthe pump light beam direction in an angle multiplexed fashion so thatthe laser beam to be amplified makes two or more passes through thepumped gain volume.
 17. The solid-state laser amplifier system accordingto claim 8 wherein said optical system is configured for directing thelaser beam to be amplified through the surface of the non-laser-activesolid along the pump light beam direction in an angle multiplexedfashion to substantially overlap with the pumped gain volume along theL₁′ direction primarily in a region where the temperature isotherms aremost parallel to the cooled surface.
 18. The solid-state laser amplifiersystem according to claim 8 wherein said optical system is configuredfor directing the laser beam to be amplified through the compositestructure at an angle substantially parallel to the surface of the laseractive material in contact with the cooler in such a way as to a zig-zagbeam path through the pumped gain volume that effectively averages outnonparallel components of the thermal isotherms experienced by differentparts of the laser beam to be amplified and thereby remove or greatlyreduce thermal aberrations on said laser beam.
 19. The solid-state laseramplifier system according to claim 8 wherein said optical system isconfigured for directing the laser beam to be amplified through thecomposite structure at an angle to the surface normal in such a way asto cancel out nonparallel components of the thermal isotherms along thesurface normal experienced by the laser beam to be amplified and therebyremove or greatly reduce thermal aberrations on said laser beam.
 20. Thesolid-state laser amplifier system according to claim 8 said opticalsystem is configured for directing the laser beam to be amplifiedthrough the composite structure at a preselected angle to the surfacenormal and for producing a zig zag beam path in a plane of the firstsurface to average out nonparallel components to thermal gradients inboth orthogonal directions to the surface normal along both directionsof W₁ and t₁.
 21. The solid-state laser amplifier system according toclaim 1 wherein the laser active solid is selected from the groupconsisting of YAG, YLF, YVO₄ and Sapphire host crystals containing laseractive atoms selected from the group consisting of Ti, Nd, Er, Yb, Cr inYAG, YLF, YVO4 and Sapphire host crystals.
 22. The solid-state laseramplifier system according to claim 8 in which the cooling device is acryogenic cooler that maintains the temperature of the laser activesolid under pumped conditions at a temperature at which a differentialchange in index of refraction of the laser active solid with temperatureis substantially close to zero (dn/dT=0) to further reduce thermalaberrations.
 23. The solid-state laser amplifier system according toclaim 10 in which the cooling device is a cryogenic cooler thatmaintains the temperature of the laser active solid under pumpedconditions at a temperature at which a differential change in index ofrefraction of the laser active solid with temperature is substantiallyclose to zero (dn/dT=0) to further reduce thermal aberrations.
 24. Thesolid-state laser amplifier system according to claim 20 in which thecooling device is a cryogenic cooler that maintains the temperature ofthe laser active solid under pumped conditions at a temperature at whicha differential change in index of refraction of the laser active solidwith temperature is substantially close to zero (dn/dT=0) to furtherreduce thermal aberrations.
 25. A solid-state laser amplifier system,comprising: a) at least one laser-active solid having dimensions lengthL₁, width W₁, and thickness t₁; b) a pumping light source; c) light beamshaping optical system positioned adjacent to the pumping light sourcefor shaping and directing a pump light beam from said pumping lightsource into a first surface of said at least one laser-active solid withan elliptical, round, or rectangular beam of light with a length or longaxis L₁′ and a width W₁′ satisfying a condition L₁′/W₁′≧1, and L₁′<L₁and width W₁′<W₁, and wherein a region of said at least one laser-activesolid illuminated by the beam of light produces a nonuniformly pumpedgain region defined by dimensions L₁′ ×W₁×t₁ of said at least onelaser-active solid giving rise to nonparallel isotherms, and wherein t₁is in a range from about 10 microns to about 1 millimetre so as to moststrongly localize the absorbed light and ensuing pumped gain region thatdevelops from pumping said at least one laser-active solid with saidpump light; d) a cryogenic cooling device, wherein the laser-activesolid is slab-shaped and is fixedly connected at a second surfacethereof to the cooling device, and wherein a major portion of heatgenerated in the laser-active solid by the pump light is removed by thecooling device to cool the second surface of the laser-active solid, andwherein said cryogenic cooling maintains the temperature of the laseractive solid under pumped conditions at a temperature at which adifferential change in index of refraction of the laser active solidwith temperature is substantially close to zero (dn/dT=0) to furtherreduce thermal aberrations; and e) an optical system configured to bringthe laser beam to be amplified into the laser active solid at an angleto a normal to the first surface of the laser active solid, so that itoverlaps with the nonparallel isotherms that arise from nonuniformpumping, and is incident at an angle to a normal to the first surface ofthe laser active solid to remove substantially nonparallel isothermsthat arise from nonuniform pumping of the laser active solid by the pumplight beam and cooling requirements to achieve high gain conditions forthe laser beam.
 26. The solid-state amplifier system according to claim25 wherein said optical system and said laser active material areconfigured to direct the laser beam to make at least two passes throughthe laser gain medium to further average the thermal aberrations andincrease energy extraction from the said gain volume.
 27. Thesolid-state laser amplifier system according to claim 25 wherein theslab-shaped laser active solid includes periodically disposed channelsof length between about 0.1 micron to about 100 microns, said channelsbeing filled with an absorbing material or scattering centers tointroduce regions of absorption and scattering losses along a directiondefined by L₁ and prevent accumulated stimulate emission along the L₁′pumped region from depleting the gain in this direction and accompanyingreduction in pulse quality for use in the amplification of laser pulses.28. The solid-state laser amplifier system according to claim 27 whereinsaid channels are small cylinders or narrow lines that run parallel to adirection of W₁′ of the pumped gain medium.
 29. The solid-state laseramplifier system according to claim 27 wherein the absorbing materialare selected from the group consisting of semi-metals that stronglyabsorb at the laser wavelength, specifically tuned quantum dots tuned toact as saturable absorbers at the laser wavelength imbedded in an indexmatching polymer.
 30. The solid-state laser amplifier system accordingto claim 27 wherein the scattering centers include air spaces or voids.31. The solid-state laser amplifier system according to claim 25including a slab-shaped nonactive laser material bonded to said firstsurface of said slab-shaped laser active solid to give a compositestructure to provide mechanical strength for thermal contact of saidslab-shaped laser active solid to the cooling device, said slab-shapednonactive laser material having an index of refraction for matching anindex of refraction of said laser active solid to avoid diffraction andFresnel losses in coupling the laser beam to be amplified into and outof the pumped gain region so that the thickness t₁ of the laser activesolid can be made as small as possible for a maximum rate of heattransfer to the cooling device so as to produce minimum thermalaberrations and maximum gain per unit length for a given power of thepumping light.
 32. The solid-state laser amplifier system according toclaim 25 said optical system is configured for directing the laser beamto be amplified through the composite structure at a preselected angleto the surface normal and for producing a zig zag beam path in a planeof the first surface to average out nonparallel components to thermalgradients in both orthogonal directions to the surface normal along bothdirections of W₁ and t₁.